Untitled DocumentModeling the mechanisms of mountain block recharge
Macro Theme Area:
Basin Scale Water Balance [Project ID: B08]
PI:
John Wilson
CO-PI(s):
N/A
Basin focus:
Regional SW
Specific area in
basin /
field sites:
Rio Grande
Summary/Goals: Water that moves through mountain blocks to adjacent basins originates as orographically enhanced precipitation and snowmelt. Traditional semi-arid zone basin groundwater models assume that basin recharge is mainly due to stream runoff from the adjacent mountain blocks, but there is also a significant subsurface contribution. Groundwater flows from the mountain block, across the mountain-bounding fault, and into the basin. This subsurface movement of water at the mountain block scale first requires that water enter the mountain block by deep percolation from the thin veneer of soil and vegetation that blankets mountain hillslopes. Mathematical modeling, existing databases, and new observations are used to simulate and understand subsurface moisture movement in the thin soil layer and the upper part of the mountain bedrock. Most water partitions upward to the atmosphere, by evapotranspiration, some moves laterally down the hill slope by gravity flow, while the remainder contributes to deep percolation of the mountain block, some of which eventually reaches the basin as mountain front recharge. The simulations and observations help us understand the controls of climate, vegetation, soil, topography and geology on deep percolation, and guide future field studies in the Rio Grande watershed.
Activities and outcomes during past year:
Without detailed modeling of the vegetation and the evapotranspiration process, the main controls on deep percolation are water availability at the bedrock-soil interface and the permeability, both matrix and fracture, of the bedrock itself. Significant deep percolation requires both available water and permeable bedrock. With equal amounts of available water, a volcanic tuff, as found in the Jemez and San Juan mountains, would have significantly greater deep percolation than a crystalline rock, as found in the Sangre de Cristo Mountains. Studies of the Alamosa Basin have noticed this significant difference in mountain front recharge, from the two different types of bounding mountains, one on the west and the other on the east. What influences water availability? Field studies and detailed modeling of a site in the Jemez Mountains suggest that a well-developed soil Bt horizon becomes an impeding layer for further downward water movement. At this site root channels also appear to provide an essentially horizontal macropore flow path associated with the low permeable soil layer. Even though this tuffaceous bedrock is highly permeable, at this site water is simply not available at the bedrock surface. Other controls on water availability, of differing importance, are slope, aspect, bedrock topography and roughness, vegetation, and climate. Effective modeling requires high resolution in time, and strongly coupled processes, including detailed modeling of soil water, evaporation, and transpiration, accounting for vegetation patterns.
Plans for the upcoming year:
The hillslope scale modeling will be integrated with Eric Small's similar 100m modeling work for basin floors. The integrated modeling study will be coordinated with the planned transect field study that will cross the mountain front of the Jemez Mountains, near Los Alamos. Data gathered in the field studies will be assimilated with the models, and be used to help parameterize the fine scale regional model that Los Alamos National Lab is developing. Concurrent with the integration of the hillslope scale modeling effort, some effort of this project will be redirected toward the installation and operation of the field instrumentation. Focus will then shift back to the modeling work as the field aspects mature.
